Acoustical impedance matching device for a high impedance transducer

ABSTRACT

A miniature acoustical impedance matching device for use with a high acoustical impedance transducer which converts electrical waves to sound waves and must be acoustically impedance matched to prevent distortion. The device includes a housing with an acoustical chamber formed therein. A first aperture is formed in the housing for allowing exit of the sound waves from the chamber, and a second aperture is formed for allowing entry of the sound waves from the transducer. The first aperture acts to inhibit exit of the sound waves for developing a predetermined sound wave pressure in the chamber. The predetermined sound wave pressure acts to provide an acoustical impedance match for the transducer to reduce or prevent distortion. A passage is coupled to the first aperture for allowing emission of the sound waves. The matching device can be mounted in a portable radio receiver, thus allowing a transducer normally operable only with a hyperbolic or exponential horn, to be used without a horn.

United States Patent [191 Payne Apr. 23, 1974 ACOUSTICAL IMPEDANCE MATCHING DEVICE FOR A HIGH IMPEDANCE TRANSDUCER [75] Inventor: Royce L. Payne, Fort Lauderdale,

Fla.

[73] Assignee: Motorola, Inc., Franklin Park, 111.

[22] Filed: June 28, 1972 [21] Appl. No.: 266,855

[52] US. Cl. 18l/0.5 R, 325/361 [51] Int. Cl G0lv 1/00 [58] Field of Search 325/352, 353, 361; 333/32,

333/34; 340/8 MM, 15; 181/27 A, 27 C, 27

D, 31 A, 0.5 R

[56] References Cited UNITED STATES PATENTS 3,249,873 5/1966 Whittemore, Jr. et al 325/361 OTHER PUBLICATIONS High Fidelity Sound Engineering -Norman H. Crowhurst -1961 Pages 197-200 Primary ExaminerRobert L. Griffin Assistant ExaminerMarc E. Bookbinder Attorney, Agent, or Firm-Vincent J. Rauner; Eugene A. Parsons 5 7] ABSTRACT A miniature acoustical impedance matching device for use with a high acoustical impedance transducer which converts electrical waves to sound waves and must be acoustically impedance matched to prevent distortion. The device includes a housing with an acoustical chamber formed therein. A first aperture is formed in the housing for allowing exit of the sound waves from the chamber, and a second aperture is formed for allowing entry of the sound waves from the transducer. The first aperture acts to inhibit exit of the sound waves for developing a predetermined sound wave pressure in the chamber. The predetermined sound wave pressure acts to provide an acoustical impedance match for the transducer to reduce or prevent distortion. A passage is coupled to the first aperture for allowing emission of the sound waves.

The matching device can be mounted in a portable radio receiver, thus allowing a transducer normally operable only with a hyperbolic or exponential horn, to be used without a horn.

2 Claims, 8 Drawing Figures ACOUSTICAL IMPEDANCE MATCHING DEVICE FOR A HIGH IMPEDANCE TRANSDUCER BACKGROUND In small, portable radio apparatus such as a receiver or a pager, to be carried by a user, it has become desirable to employ transducers operating with hyperbolic or acoustical horns for reproducing the audio signals. One such system is described in co-pending U.S. Pat. application Ser. No. 216,674, now US. Pat. No. 3,748,583 filed Jan. 20, 1972, and assigned to the same assignee as this application. In that application, the acoustical horn described provides between and (db) of acoustical, or sound level gain for the sound waves developed by the transducer.

In many instances the higher level audio output provided by an exponential or hyperbolic horn is not necessary. In these instances, the transducer output level itself is sufficient. As the radio apparatus housing is normally made from a plastic material which is injection molded, it becomes a relatively simple matter, to modify the housing mold for eliminating that part of the housing which forms the acoustical horn. In fact, both housings may be made with an adjustable mold. Of course it would be most desirable to use the same receiver chassis and transducer in all varieties, both low and high audio level of the radio apparatus. The transducer and acoustical horn are, however, matched to one another in order to provide the high level audio output without substantial audio distortion. That is, the length of the acoustical horn causes a build-up of sound pressure waves. This build-up of sound pressure waves acts to load the transducer and provide a proper acoustical impedance match, thus preventing distortion of the audio signal as a result of improper acoustical matching of the transducer. If the transducer output is coupled directly to a point external of the housing, the absence of a proper sound pressure level loading on the transducer will result in an improper acoustical impedance match for the transducer. The improper acoustical impedance match will cause the transducer to produce severely distorted audio signals.

SUMMARY It is an object of this invention to provide a miniature acoustical impedance matching device for use with a high acoustical impedance transducer which properly matches the transducer and prevents distortion.

Another object of this invention is to provide a miniature acoustical impedance matching device which allows one transducer to be used with and without a matched acoustical horn in accordance with the audio power level desired.

Yet another object of this invention is to provide an improved radio receiver apparatus which includes an acoustical impedance matching device for properly matching and loading a high acoustical impedance transducer.

In practicing this invention, a miniature acoustical impedance matching device is provided for use with a high acoustical impedance transducer that converts electrical waves to sound waves and must be acoustically impedance matched. The transducer is normally coupled to an acoustical horn which acts to provide the necessary acoustical loading and impedance matching for the transducer. Proper matching and loading of the transducer by the horn prevents distortion of the audio signals by the transducer. The horn also acts as an amplifier for amplifying the audio signals. The miniature acoustical impedance matching device includes a housing which has an acoustical chamber defined therein that has a first cross-sectional area. An aperture is formed in the housing for allowing exit of the sound waves from the chamber. The aperture has a second smaller cross-sectional area than the area of the chamber, thus inhibiting exit of the sound waves. This results in the development of a predetermined sound wave pressure in the chamber. The sound wave pressure acts to load and provide an acoustical impedance match for the transducer, thus preventing distortion of the reproduced audio signals.

The miniature acoustical impedance matching device may be employed in a portable radio receiver for properly matching the transducer employed therein. This allows the same radio chassis, including the same transducer device, to be used, in applications where a high audio output level is required, and where a lower audio output level is required.

THE DRAWINGS FIG. 1 is a perspective view of a portable radio pager including the acoustical impedance matching device of this invention; and also of the acoustical horn portion of a housing which may be provided when higher audio levels are desired and for which this invention has been substituted.

FIG. 2 is a section view of the pager of FIG. 1 alongthe section lines AA, showing the circuit board and components of the removable radio receiver mounted in the pager housing;

FIG. 3 is a view of the portable radio receiver chassis, including the transducer adapted to be mounted in the pager housing shown in FIG. 1;

FIG. 4 is a perspective view of the housing shown in FIG. 1 with the U-shaped front and rear cover and removable radio receiver chassis removed;

FIG. 5 is a cross-sectional view of the housing in FIG. 4 along the section lines BB;

FIG. 6 is a perspective view of the acoustical impedance matching device of this invention showing the interior construction;

FIG. 7 is a perspective view of the coupling device shown in FIG. 2;

FIG. 8 is a perspective view of the transducer device shown in FIG. 3 with its cover removed.

DETAILED DESCRIPTION Referring to FIG. 1, a rectangular elongated housing 10 contains a self-contained removable radio receiver chassis. Clip 11 secured to back wall 12 of U-shaped housing cover 17, allows the radio pager to be secured in a users shirt pocket or on the users belt. Top wall 23 of U-shaped housing cover 17, and inside top wall 13 of housing 10 each have an aperture 14 therein, through which extends switch 15 on the radio receiver chassis. Apertures 16 allows exit of the audio sound waves from housing 10.

In many applications it is desirable to provide an acoustical horn for amplification of the audio signals developed by the transducer in the pager. In FIG. I, an acoustical horn 20 such as disclosed in the above-noted co-pending patent application is shown. When such an acoustical horn is used, side wall 18 becomes an interior partition wall. Aperture 19 in side wall 18 couples the sound waves from the transducer to acoustical born 20. Sound waves coupled to acoustical horn 20 from aperture 19 are amplified and exit from aperture 21 in top wall 22.

If a radio pager is desired which does not have an amplified audio output, the radio pager housing will not include acoustical horn section 20. Side wall 18 will be an outer wall and aperture 19 will not be provided. The audio output will be coupled from the transducer through an acoustical impedance matching device, to be further described in a subsequent portion of this application, and will exit at apertures 16 in housing 10 as noted above.

Referring to FIGS. 2 and 3, the portable radio receiver chassis for the pager is shown. Printed circuit board 25 seats in housing 10. Switch is mounted thereon and extends through aperture 14 in top wall 13. A number of components 26 representative of the entire radio circuitry for the radio receiver chassis are shown on printed circuit board 25. A battery 27 is secured in battery holder 28 on printed circuit board 25. Battery 27 provides the power necessary for operating the radio receiver. Transducer device 29 is also secured to printed circuit board and converts the received demodulated signals to audio signals, thus providing an entirely self-contained, operable, portable radio receiver chassis.

Referring to FIG. 8, there is shown transducer device 29 with housing cover 31 removed. The transducer 35 is located beneath the structure shown and directly below aperture 36. The audio signals developed by transducer 35 are coupled through the spiral passage shown at 32 and emitted at aperture in transducer housing cover 31 as sound waves. These sound waves are coupled via a coupling device 33, (shown in FIGS. 2 and 7) to the acoustical impedance matching device 34 secured to housing 10.

Referring to FIG. 4, the acoustical impedance matching device 34 is more clearly shown. Device 34 is secured to top wall 13, rear wall 37, and side wall 18. Device 34 has a first aperture 38 in the top surface thereof. Coupling device 33 is seated in aperture 38, and held in place by the aperture 38 side walls.

Coupling device 33 is a resilient cylindrical member, which has an outer wall 40, and an aperture 41, axially extending through the cylinder. Circumferential groove 42 is cut in outer wall 40 of coupling device 33, adjacent end 43. The side walls of aperture 38 on matching device 34 are inserted into groove 42 for securing coupling device 33 to matching device 34. Coupling device 33 may be manufactured from rubber or a synthetic equivalent of rubber.

Referring again to FIG. 2, output aperture 30 of transducer device 29 is positioned adjacent aperture 41 in coupling device 33. When circuit board 25 is inserted into housing 10, coupling device 33 is compressed, with end 44 bearing against transducer housing cover 31 and surrounding aperture 30. Coupling device 33 acts to couple the sound waves from output aperture 30 of transducer device 29 to acoustical impedance matching device 34. Because coupling device 33 only resiliently bears against transducer housing 31, the portable radio receiver may be removed and reinserted in housing 10, and when reinserted will make a good acoustical connection with impedance matching device 34. This allows printed circuit board 25, and the entire self-contained radio receiver chassis mounted thereon,

to be removed, tested and easily reinserted; or to be inserted in a number of housings for use with different types of acoustical horns or impedance matching devices.

As previously mentioned, transducer device 29 is designed to normally work through a coupling chamber into an acoustical horn shown at 20 in FIG. 1. If the audio sound wave amplification produced by acoustical horn 20 is not desired, a device must be used which will properly load transducer 29 so as to prevent it from causing a distortion of the reproduced audio signals. Acoustical impedance matching device 34 provides this necessary loading and acoustical impedance matching function.

Referring to FIG. 6, sound waves developed by transducer 29 are coupled through coupling device 33 to aperture 38 in impedance matching device 34. Impedance matching device 34 includes a top wall 50, side walls 51, 52, 53 and 54, and may include a bottom wall. In the embodiment shown, the bottom wall for impedance matching device 34 is supplied by bottom wall 37 of housing 10.

An acoustical chamber 56 is defined by the walls of device 34, bottom wall 37 of housing 10, and an inner partition wall 58 of matching device 34. Sound waves coupled through aperture 38 enter acoustical chamber 56. Acoustical chamber 56 has a cross-sectional area which is substantially greater than aperture 38. Side wall 53, inner partition wall 58, top wall 50 and bottom wall 37 of housing 10 also define an output aperture 60. The cross-sectional area of output aperture 60 is substantially smaller than the cross-sectional area of acoustical chamber 56. Sound waves entering acoustical chamber 56 via aperture 38 will attempt to exit via aperture 60. Because the cross-sectional area 60 is substantially less than the cross-sectional area of chamber 56, aperture 60 will act as a restriction that will inhibit exit of the sound waves. This will result in a predetermined sound wave pressure developing in chamber 56. The sound wave pressure will be reflected back to transducer 29 where it will act to load transducer 29 and provide an acoustical impedance match which will prevent the transducer from distorting the reproduced audio signals.

A second, inner partition wall 61, along with side wall 53, top wall 50, and bottom wall 37 of housing 10 acts to provide a passage 62 for coupling the sound waves from aperture 60 to a point external of housing 10 via aperture 16. Passage 62 has at the junction of first inner partition wall 58 and second inner partition wall 61, the same cross-sectional area as aperture 60. The cross-sectional area of this passage increases in an ex ponential fashion until it reaches output aperture 63 and side wall 54.

As can be seen, a miniature acoustical impedance matching device has been provided for use with a high acoustical impedance transducer which properly matches the transducer and prevents distortion. This allows one transducer to be used with and without a matched acoustical horn in accordance with the audio power level desired thus allowing variations in the housings of radio pagers while using the same radio receiver chassis and audio transducer.

I claim:

1. A miniature acoustical impedance matching device for use with a high acoustical impedance transducer means which converts electrical waves to sound waves and which must be impedance matched to prevent distortion, said device including in combination;

a. a housing having top, bottom and side walls defining an acoustical chamber therein having a first cross-sectional area; 5

b. a first sound wave restricting aperture in said housing defined by said housing walls for allowing exit of said sound waves from said chamber, said first aperture having a second cross-sectional area smaller than said first cross-sectional area for restricting free exit of said sound waves, said acoustical chamber developing a predetermined sound wave pressure therein in responses to said first aperture restricting free exit of said sound waves, and said sound pressure providing an acoustical impedance match for said transducer means;

c. passage means coupled to said first aperture for allowing emission of said sound waves, said passage means being defined by said housing walls and having a first end coupled to said first aperture and having substantially the same cross-sectional area as said first aperture and said passage means further having an exponentially expanding crosssectional area; and

d. means for coupling said sound waves from said transducer means to said chamber including a second aperture formed in said housing.

2. A miniature acoustical impedance matching device as claimed in claim 1 wherein said coupling means includes compressible coupling means mounted to the walls of the second aperture in the matching device, said coupling means being compressed by said transducer means between said transducer means output opening and the acoustical chamber for coupling the sound waves to said acoustical chamber. 

1. A miniature acoustical impedance matching device for use with a high acoustical impedance transducer means which converts electrical waves to sound waves and which must be impedance matched to prevent distortion, said device including in combination; a. a housing having top, bottom and side walls defining an acoustical chamber therein having a first cross-sectional area; b. a first sound wave restricting aperture in said housing defined by said housing walls for allowing exit of said sound waves from said chamber, said first aperture having a second cross-sectional area smaller than said first cross-sectional area for restricting free exit of said sound waves, said acoustical chamber developing a predetermined sound wave pressure therein in responses to said first aperture restricting free exit of said sound waves, and said sound pressure providing an acoustical impedance match for said transducer means; c. passage means coupled to said first aperture for allowing emission of said sound waves, said passage means being defined by said housing walls and having a first end coupled to said first aperture and having substantially the same crosssectional area as said first aperture and said passage means further having an exponentially exPanding cross-sectional area; and d. means for coupling said sound waves from said transducer means to said chamber including a second aperture formed in said housing.
 2. A miniature acoustical impedance matching device as claimed in claim 1 wherein said coupling means includes compressible coupling means mounted to the walls of the second aperture in the matching device, said coupling means being compressed by said transducer means between said transducer means output opening and the acoustical chamber for coupling the sound waves to said acoustical chamber. 